The present invention relates to a device associated with a copier, printer or like machine for sensing lateral dislocation and skew of a sheet such as an original document or a copy sheet which is being transported toward a predetermined position in the machine.
An automatic document feeder (ADF) installed in a prior art electrophotographic copier automatically feeds a document to a predetermined position on a glass platen by means of belts. In the automatic document feeder, at least two sensors are positioned face-to-face at laterally opposite sides of a sheet transport path in order to sense passage of the document being transported. Skew of the document on the transport path is sensed in terms of a difference between the times when the document moves past the sensors, e.g. difference between the times when the sensors have started sensing the document in movement. Also installed in the prior art automatic document feeder are an array of parallel sensors which are arranged in a direction perpendicular to an intended direction of document feed, that is, laterally with respect to the transport path. This parallel sensor array functions to check for any lateral displacement of the document in the transport path by sensing that the number of the sensors which detected the passage of the document differs from a predetermined number, i.e. the number of the sensors which sense a document in a proper transport condition. An automatic document feeder with such a function of sensing a document transport condition is quite useful and has been extensively used.
However, a problem has been encountered with the above-described described prior art device in that independent sensors have to be employed in order to sense at least the skew and lateral dislocation of a document and, moreover, a plurality of sensors are required for each of the skew and lateral dislocation.